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The two-dimensional (2D) hexagonal boron nitride (h-BN) nanosheet known as "white graphene" has great promise for applications in nanoelectronics and optoelectronics. Recent experimental fabrication of h-BN nanoribbons (BNNRs) have added to interest in the material. Now numerical studies show how line defects in boron nitride nanoribbons affect the electronic and magnetic properties in ways that may benefit applications using the material.

The h-BN sheet is a good complementary material to the semimetal graphene because it is a wide band semiconductor with a direct energy gap of about 4.8 eV. Graphene has a zero band-gap, which greatly limits its practical applications in nanoelectronics. Topological defects, such as a line defect or grain boundary, are inevitable when producing 2D materials by chemical vapor deposition (CVD). In Nanotechnology researchers at Nanjing University in China have reported fascinating electronic and magnetic properties caused by a new type of line defect using first-principles calculations.

A new type of line defect

Researchers had previously proposed a new kind of line defect where the hexagons of the honeycomb lattice are replaced by square-octagon (4|8) pair rings denoted here as LD-(4|8)-BNNR. This has a lower energy than the traditional pentagon-heptagon (5|7) pair defect because it has no bonds between atoms of the same element.

Band gap dependence

The researchers at Nanjing studied different types of BNNR, including nanoribbons with an armchair- or zigzag-shaped edge, and nanoribbons with edges terminated with hydrogen. Interestingly, the team found that the band gaps of the bare and fully-hydrogen terminated armchair edge BNNR with an asymmetric (4|8) line defect show a specific type of behaviour. Considering the BNNR in terms of two parts either side of the line defect, the properties depend only on the ribbon width of its narrower part, but not the whole BNNR's width. This is in contrast to what is usually seen for armchair-edge graphene nanoribbons and pristine BNNRs without the line defects. Both were also found to be non-magnetic semiconductors, having an indirect and direct band gap, respectively.

Magnetic properties

The team also found that the zigzag edge LD-(4|8)-BNNR with both edges N-terminated had two degenerate ground states. One state is half-metallic ferromagnetic (FM) and the other is metallic antiferromagnetic (AFM). However, with both edges B-terminated the BNNR is an AFM insulator with an indirect band gap.

These numerical results present a deep understanding of the electronic and magnetic properties of LD-(4|8)-BNNRs. They also highlight the promise of using them to advantage in nanoelectronics, spintronics and electromechanical devices.

About the author

Yang Han is a PhD candidate in the Department of Physics, Nanjing University in China, under the supervision of Prof. Jinming Dong. Her current research focuses on the confined growth and physical properties of low-dimensional nanomaterials by molecular dynamics simulations and first-principles calculations.